The Affordable Care Act is the biggest new health care program in decades, but the Obama administration has ruled that neither the federal insurance exchange nor the federal subsidies paid to insurance companies on behalf of low-income people are “federal health care programs.”

The surprise decision, disclosed last week, exempts subsidized health insurance from a law that bans rebates, kickbacks, bribes and certain other financial arrangements in federal health programs, stripping law enforcement of a powerful tool used to fight fraud in other health care programs, like Medicare.

The main purpose of the anti-kickback law, as described by federal courts in scores of Medicare cases, is to protect patients and taxpayers against the undue influence of money on medical decisions.

Kathleen Sebelius, the secretary of health and human services, disclosed her interpretation of the law in a letter to Representative Jim McDermott, Democrat of Washington, who had asked her views. She did not explain the legal rationale for her decision, which followed a spirited debate within the administration.

Under the Affordable Care Act, millions of people will be able to buy insurance from “qualified health plans” offered on exchanges, or marketplaces, run by the federal government and by some states.